Defluorination of phosphate rock



contents of the phosphate rock.

2,839,361 DEFLUORINATION F PHOSPHATE ROCK 2 Claims. (Cl. 23-108) Thisinvention relates to the defluorination of phosphate rock and similarnatural phosphate materials, and has for its object the provision of animproved method of defluorinating such materials by calcination.

In our copending application for Letters Patent of the United States,Serial No. 213,284, filed Feb. 28, 1951, we have described and claimed amethod of deiiuorinating phosphate rock by calcination in the presenceof the reaction product of sodium carbonate (Na CO and phosphoric acid(H PO under certain prescribed conditions involving (among otherconditions) the ratio of sodium oxide (Na O) to phosphorus pentoxide (P0 in the reaction product and the grade (P 0 content) and the lime (CaO)and silica (SiO We recognize in that application that some sodiumcompounds other than the carbonate (e. g. the bicarbonate (NaHCOhydroxide (NaOH) etc.) react with phosphoric acid in equivalent manner.We have now made the surprising and entirely unexpected discovery thatwhen phosphate rock is calcined at a temperature exceeding 2600" F. inthe presence of water vapor and a mixture of sodium chloride (N aCl) andphosphoric acid, under substantially the same conditions as prescribedin our aforementioned application, substantially complete defluorinationcan be effected without objectionable fusion or sintering of thecalcining charge. The present invention is based on that discovery, andinvolves defluorinating phosphate rock by calcination at a temperatureof at least 2600 F. without substantial fusion in the presence of watervapor and of a reagent mixture consisting essentially of sodium chlorideand phosphoric acid, with a mol ratio of sodium to phosphorus(calculated as Na O and P 0 respectively) in the mixture between 1.6 and2.8, and with such relative amounts of (3210, Na O, P 0 and SiO in thecombined phosphate rock and reagent mixture that the mol ratio of theseconstituents is between 1.3 and 2.8 (and preferably between 1.6 and 2)in the molar formula MOlS CaO +Na2O-3P O5 M015 516 Since part, at leastof the efficacy of the sodium chloride in imparting refractoriness tothe calcining charge is believed to be due to the sodium oxide thatresults from the decomposition of the chloride during calcination in thepresence of water vapor, the ratio of sodium to phosphorus in thereagent mixture is in practice considered a ratio of Na O to P O and isherein so referred to.

It is essential that the silica content of the calcining charge (i. e.the combined phosphate rock and reagent mixture) be within the range of2 to 6% (by Weight on a dry basis) determined, as customary in thephosphate industry, as insoluble matter (insol.). The silica (or insol.)content of the rock then determines the relative United States Patent *92,839,361 Patented June 17, 1958 ICC 2 proportions of CaO, Na O and P 0in the calcining charge, in accordance with the aforesaid molar formula.Additionally, it is essential that the molar ratio of Na O to P 0 in thereagent mixture added to the rock be within the range of 1.6 and 2.8.

In plant practice it is frequently more convenient to determine andexpress the ratio of Na O to P 0 in the reaction mixture in terms ofactual weights, and so expressed the mol ratio (Na O to P 0 of 1.6 to2.8 becomes the weight ratio of 0.7 to 1.2. A convenient and preferredweight ratio of Na O to P 0 in the reagent mixture is around 1,corresponding to a 1:001 ratio of about 2.3. When the calcining chargeis made up of approximately 93% of phosphate rock (containing about 35%P 0 about 50% CaO and from 4 to 5% insol) and 7% of added Na O and P 0in the form of a reagent mixture having a Na O/P O weight ratio of 1(all percentages being by weight on a dry basis), the mol ratio of theconstituents of the, calcining charge in the aforementioned molarformula will be approximately 1.6 to 2.0.

In practicing the invention on run-of-mine rock of fairly constant grade(i. e. P 0 content) but of variable silica content, it is convenient andpractical to determine the composition of the reagent mixture on thebasis of the insol (silica) content of the rock, rather than on thebasis of the aforementioned molar formula. Thus, with a phosphate rockrunning around 35% P 0 (say 33 to 36% P 0 the composition of the reagentmixture can be determined from the following table on the basis of theinsol. content of the rock:

Broad range Preferred range Percent lnsol Wt. Ratio, Moi Ratio. Wt.Ratio. M01 Ratio, ew/ 205 Na20/P205 NH20/P20t NazO/Pzoa It will be seenthat as the silica (insol.) content of the rock increases from 2 to 6%,the preferred rrrol ratio of Na O/P O increases from 1.90 to 2.50; anincrease of 0.15 for each increase of 1% in the silica content of therock. For the ratio to thus increase, the P 0 (in the reagent mixture)must decrease in relation to the Na O, and this decrease in F 0 will beproportional to the increase in the silica content of the rock. Thus, bymaking up the reagent mixture from the foregoing table the criticalmolar formula range of the invention is conveniently attained in plantpractice.

Where the silica (insol.) content of the run-of-mine rock (containing33-36% P 0 is fairly constant between about 3% and 3.5%, the criticalratios of the invention may conveniently be attained by use of thefollowing table; the determining factor being the amount of P O added inthe reaction mixture expressed in percent by weight on the combinedweight of the rock and the Na O and P 9 in the reagent mixture, allweights being on a dry basis. Thus, with 5% of added P 0 the added Na Owill be 4.2% (i.e. 5 .84), and the balance of the calcining charge(90.8% by weight) will be phosphate rock. The amount of added P 0 isdetermined to a large extent on the current availability of phosphoricacid, and preferably is between 5 and 6%. As indicated in the table,increasing the amount of added P 0 decreases the Na O/P O ratio. Thecurrently desired by the plant analytical laboratory.

In practicing the invention in a rotary kiln, the phosphate rock isintroduced into the feed or cold end of the kiln along with the reagentmixture. The reagent mixture is conveniently prepared by dissolving a:predetermined amount of any commercially available form of sodiumchloride or' common salt in a predetermined amount of an aqueoussolution of crude phosphoric acid in a suitable reaction tank withconstant stirring. The reaction tank and other equipment for handlingthe reagent mixture is rubber-lined, or otherwise suitably protectedagainst the corrosive effects of dilute phosphoric acid. Thus, thesodium chloride may be dissolved in the excess water of a relativelydilute aqueous solution of phosphoric acid, such as an unconcentratedwet process phosphoric acid. When practicing the invention with thepreferred Na O/P O weight ratio of 1, the maximum concentration of P inthe phosphoric acid solution is about 15% in order that the amount ofsodium chloride required to give the preferred weight ratio can bedissolved in the excess water of the phosphoric acid solution. Theconcentration of the chemical compounds in the resulting reagent mixturewill be around 23 Since the phosphate rock customarily contains 1.0 to14% moisture, when the aforesaid reagent mixture is'mixed therewith, atthe feed end of the kiln, there results a slurry containing 30' to 40%of water.

The phosphate rock may be of any of the usual commercial products of aphosphate rock mill or concentrator. The particle size of the rock isnot especially critical, and a larger proportion of fine particle sizematerial may be included in the calcining charge, as contrasted withother defluorinating processes in which rock of time particle size istroublesome. Thus, in practicing the invention, it is possible toinclude in the calcining charge substantial amounts of froth flotationconcentrate (mostly minus 20 mesh and plus 150 mesh), and plantconcentrate (combined belt and froth flotation concentrates, mostlyminus 14 mesh and :plus 150 mesh). Generally speaking, it is desirablein deiluorinating processes in which phosphoric acid is included in thecalcining charge to use as much fine particle size rock as possible, andthe possibility of doingso is an advantage of the invention. it has beenfound that a coarse feed (e. g. washer plant screen product) is usuallydischarged from a rotary kiln with a finer particle size than the feed,while a fine feed (e. g. froth flotation concentrate) is usuallydischarged with a coarser particle size than the feed.

Calcination is most conveniently carried out in a rotary kiln, althoughother types of calcining equipment may be used. Calcination is conductedin the presence of water vapor, care being exercised to assure intimateand continuous association of water vapor with the entire body of thecharge until substantially complete defluorination is effected. Thecalcining temperature should ultimately be sutficiently high toeliminate substantially all of the fluorine and to impart highfertilizer availability to the phosphate content of the calcinedproduct. and to this end should be at least 2600 F. and may be as highas 2800 F. Calcination is carried out in the absence of substantialfusion or sintering of the charge. A detention. period of to 20 minutesat approximately the ultimate calcining temperature, e. g. the hot zone'of a rotary kiln, is suficient to substantially defiuorinate the rockand impart high fertilizer availability to its phosphate content.

In a rotary kiln, the depth of charge should be such as to insureadequate penetration of water vapor and escape of evolved fluorine. Toodeep a bed of charge impairs these requirements and results in poordefluorination. Where calcination is carried out in a single passthrough a rotary kiln 6 to 8 feet in diameter and 120 to 250 feet inlength, rotating at a speed of from 15 to 50 seconds per revolution, afeed rate or" 1 to 10 tons per hour gives a satisfactory depth of chargefor eifective defiuorination.

Calcination may be carried out in two stages. The first stage, forconvenience called the calcining burn, may be carried out in arelatively short rotary kiln, e. g. 60-140 feet, at a temperature of2000-2500 F. with a rate of feed up to tons per hour. The second stage,for convenience called the defluorinating burn, may then be carried outin a much longer kiln, e. g. 140-250 feet,

0 at a temperature of 26002800 F. with a feed rate up to 10 'tons perhour. In plant practice it is customary to speak of these stages aspasses, i. e. first pass and second pass. The lumps and cakes of thefirst pass calcine or clinker are crushed to mostly minus ,4 inch (3mesh), and the crushed: clinker is fed dry to the second pass.

The following example illustrates a practice of the invention on acommercial scale in two passes:

1 st pass operation Feed (unground concentrate):

Percent phosphate rock 91.44 Percent P 0 (added as H PO 4.41 Percent NaO (added as NaCl) 4.15

The Na O/P O weight ratio in the reagent mixture was 0.94. The molarformula ratio of the calcining'charge (feed) was 2.2.

Chemical analyses The first pass was con-ducted in an 8 x rotary kiln ata temperature of about 2200 F. in the hot zone. The feed rate was about10 tons per hour.

2nd pass operation The 2nd pass operation was conducted in a 6 X rotarykiln at a temperature of about 2700 F. The first pass clinker wasscreened and crushed to minus 4" before feeding to the 2nd pass kiln.The feed rate was about 2.9 tons .per hour. A typical clinker analysisis given below:

Percent P 0 39.90 'Percent CaO 50.20 Percent Na o 4.60 Percent F 0.08

P 0 available:

0.4% HCl 39.75 2% citric 37.70 Ne'u; amm. citrate 37.30

The principal advantage of carrying out the calcination in two passes isthat the soft cakes and lumps, which are'formed at the relatively lowtemperature ot'the first pass, are easily broken down by mechanicalcrushers before going to the second pass where the temperature employedwould otherwise cause the cakes and lumps to fuse or glaze to such adegree that satisfactory delluorination cannot take place. Once thesecakes and lumps are broken down they have little tendency to reform,conse quently a higher feed rate can be employed in the second pass thanin a single stage operation in the same kiln. The increased outputresults in a lower unit cost of the finished product. Since the firstpass greatly reduces the tendency of the charge to form or re" andlumps, the operation in the second pass, which is the dcfluorinatingburn, is much smoother. In other words, most of the operatingdiificulties are confined to the first pass or calcining burn, wheretheir deleterious effects are of small if any practical significance,and the critical second pass or defiuorinating burn may consequently becarried out with almost complete freedom from operating difliculties.

Contrasted with the method of our aforementioned application Ser. No.213,284, in which calcination is carried out in the presence of thereaction product of sodium carbonate and phosphoric acid, the method ofthe present invention has the following advantages:

(1) Sodium chloride is cheaper than soda ash, and the reagent cost isthereby reduced about $3.50 per ton of final marketable calcine.

(2) The calcining charge is capable of withstanding a higherconcentration of fluorine in the gaseous atmosphere in contacttherewith, resulting in increased output and hence lower cost per ton offinal calcine.

(3) The calcining charge is more amenable to defluorination in one ofthe two coordinated stages of calcination in which a fluorine-enrichedphosphatic material is defiuorinated, with fluorine recovery, in theother stage, disclosed in Hollingsworths copending application forLetters Patent of the United States Ser. No. 314,842, filed Oct. 15,1952, now Patent No. 2,753,253.

(4) Sodium chloride can be directly dissolved in unconcentratedphosphoric acid as received from the acid plant.

(5) Less tendency to sinter, form balls and rings in the kiln.

(6) In view of the less tendency to sinter, the need and advantages oftwo stage calcination are minimized, and elimination of the first passbecomes possible in more operations.

There is in the present method an increase in corrosive gases since somehydrochloric acid (HCl) will be present along with the hydrofluoric acid(HF). If the gaseous product of the kiln operation is discharged to theatmosphere, these corrosive gases must be neutralized, as for example byblowing powdered limestone into the base of the stack. 0n the otherhand, where the plant is equipped to recover fluorine from the gaseousproduct of the defluorination calcination, it is easy to also recoverand market the hydrochloric acid. The presence of hydrochloric acid doesincrease kiln corrosion, especially in the 6 first pass kiln, but on theother hand the presence of hydrochloric acid appears, to some extent atleast, to have a beneficial effect on defluorination.

Throughout this specification and the appended claims, sizing isexpressed in terms of Tyler standard screen-scale sieves, and withoutsubstantial fusion means in the absence of such fusion or sintering asto cause the charge to become sticky, in whole or in part, and tend tocling or stick to the wall of the calcining apparatus, and, in a rotarykiln, to ball-up and to fail to flow freely and easily through the kiln.By substantially defluorinated is meant a phosphate product containingless than 1 part of fluorine per parts of phosphorus. The proportioningof the phosphate rock and reagent mixture in making up the calciningcharge, herein variously specified, is to be understood as applying tothe charge as initially fed to the calcining apparatus.

We claim:

1. The method of defluorinating phosphate rock which comprisessubjecting the rock with a silica content of from 2 to 6% to calcinationat a temperature of at least 2600 F. without substantial fusion in thepresence of water vapor and of a reagent mixture consisting essentiallyof sodium chloride and phosphoric acid, the mol ratio of the sodium andphosphorus contents (calculated as Na O and P 0 respectively) of thereagent mixture being between 1.6 and 2.8 and the Cat), Na O, P 0 andSi0 content of the combined phosphate rock and reagent mixture beingsuch that the mol ratio of these constituents in the molar formula MolsCaO+Na 03P 0 Mols SiO is between 1.3 and 2.8, and maintaining thecalcining charge at said calcining temperature for a sufiicient periodof time to produce a phosphate product having high fertilizeravailability and containing less than one part of fluorine for each 100parts of phosphorus.

2. The method of claim 1 in which calcination of the phosphate rock andreagent mixture is carried out in two stages each without substantialfusion and in the presence of water vapor, the first stage ofcalcination being carried out at a temperature between 2000 and 2500 F.and the clinker from the first stage of calcination being crushed tomostly minus 3 mesh, and the crushed clinker being subjected to thesecond stage of calcination at a temperature of at least 2600 F.

References Cited in the file of this patent UNITED STATES PATENTS1,878,185 Roethe Sept. 20, 1932 2,093,176 Tromel Sept. 14, 19372,288,112 Shoeld June 30, 1942 2,337,498 Ritter et a1. Dec. 21, 19432,442,969 Butt June 8, 1948 2,556,541 Hollingsworth June 12, 1951

1. THE METHOD OF DEFLUORINATING PHOSPHATE ROCK WHICH COMPRISESSUBJECTING THE ROCK WITH A SILICA CONTENT OF FROM 2 TO 6% TO CALCINATIONAT A TEMPERATURE OF AT LEAST 2600*F. WITHOUT SUBSTANTIAL FUSION IN THEPRESENCE OF WATER VAPOR AND OF A REAGENT MIXTURE CONSISTING ESSENTIALLYOF SODIUM CHLORIDE AND PHOSPHORIC ACID, THE MOLRATIO OF THE SODIUM ANDPHOSPHORUS CONTENTS (CALCULATED AS NA2O AND P2O5 RESPECTIVELY) OF THEREAGENT MIXTURE BEING BETWEEN 1.6 AND 2.8 AND THE CAO, NA2O, P2O5 ANDSIO2 CONTENT OF THE COMBINED PHOSPHATE ROCK AND REAGENT MIXTURE BEINGSUCH THAT THE MOL RATIO OF THESE CONSTITUENTS IN THE MOLAR FORMULA